1. Field of the Invention
The present invention relates to a blue colored composition for a color filter, which is used for forming a color filter for use in a color liquid crystal display device and a color image pickup tube element, and a color filter formed using the same.
2. Description of the Related Art
A liquid crystal display device has a liquid crystal layer provided between two polarizers. The liquid crystal layer controls a polarization degree of light having passed through the first polarizer, controlling an amount of light passing through the second polarizer, thereby effecting the display. Presently, a liquid crystal device using a twistednematic (TN)-type liquid crystal is prevailing. The liquid crystal display enables color display by providing a color filter between two polarizers. Since the device has been used in a television or a personal computer monitor in recent years, a color filter is increasingly required to have a high luminance and a high contrast.
A color filter includes fine stripe filter segments having two or more different hues arranged in parallel to or intersected with each other on the surface of a transparent substrate such as glass, or fine stripe filter segments horizontally and vertically arranged at a constant sequence. The filter segments are fine and the width is several microns to several hundred microns, and they are regularly arranged at a predetermined sequence by every hue.
In general, a transparent electrode for driving liquid crystal is formed on the color filter by vapor deposition or spattering in a color liquid crystal display device. Further, an alignment film for aligning the liquid crystal in a predetermined direction is formed over the transparent electrode. In order to make the best use of the performances of these transparent electrode and alignment film, the formation of them is required to be carried out at a high temperature of usually 200° C. or more, and preferably 230° C. or more. Accordingly, the color filter is presently prepared mainly by a method called a pigment dispersion method wherein a pigment, which is excellent in light resistance and heat resistance, is employed as a colorant.
However, generally, a color filter with a pigment dispersed therein has a problem that scattering of light due to a pigment disturbs a polarization degree controlled by a liquid crystal. Specifically, light is likely to be leaked when light must be shut out (OFF state) and transmitted light is attenuated when light must be transmitted (ON state). Thus, a ratio of luminances (contrast ratio) on a display device in the ON state and the OFF state becomes low.
In order to realize a high luminance and a high contrast of a color filter, a pigment to be contained in a filter segment has been hitherto subject to finely-dividing treatment. For example, Jpn. Pat. Appln. KOKAI Publication No. 2001-264528 discloses preparing a red pigment by pouring a solution of a diketopyrrolopyrrole pigment in a concentrated sulfuric acid into water to precipitate a pigment. Then, a mixture containing the precipitated pigment, a pigment derivative, a water-soluble inorganic salt, and a water-soluble organic solvent which does not substantially dissolve this water-soluble inorganic salt is kneaded and then the water-soluble inorganic salt and the organic solvent are removed from the kneaded mixture.
However, even when a pigment (obtained by subjecting particles called crude having a particle diameter of 10 to 100 μm as prepared by a chemical reaction to pigmentation treatment to form a mixture of primary particles and secondary particles which are an aggregates of primary particles) is simply finely-divided by various finely-dividing treatment methods, a pigment in which finely dividing of primary particles or secondary particles has been progressed is generally easily aggregated and, when finely dividing has progressed too much, a huge massy pigment solid is formed. Further, when a pigment in which finely dividing has progressed is dispersed in a pigment carrier such as a resin, and is tried to be stabilized by putting secondary particles of a pigment as close to primary particles as possible again, it is very difficult to obtain a stable colored composition.
A colored composition obtained by dispersing a pigment, in which finely dividing has progressed, in a pigment carrier often becomes to have a high viscosity due to aggregation of pigment particles over time, exhibiting thixotropic properties. Such an increase in a viscosity and deficiency in flowability of a colored composition cause a problem on manufacturing and various problems on a product value. For example, formation of a filter segment of a color filter is generally performed by spin-coating a colored composition in which a pigment is dispersed in a pigment carrier containing a monomer and a resin, on a glass substrate. However, when a colored composition having a high viscosity and poor flowability is used, a coated film having a uniform thickness can not be obtained due to deteriorated spin coating properties and deteriorated leveling.
Since the target spectroscopic spectrum is not generally obtained by single pigment, each color pixel of RGB (red, green, blue) forming a color filter is formed by using a colored composition containing two or more kinds of pigments in many cases. However, different kinds of pigments easily cause heterogeneous aggregation, and when two or more kinds of pigments are contained, it is particularly difficult to obtain a stable colored composition. For example, a copper phthalocyanine blue pigment excellent in light resistance and heat resistance is generally used in a colored composition for use in forming a blue filter segment. However, since it is difficult to obtain a desired spectroscopic spectrum by a copper phthalocyanine blue pigment alone, a spectroscopic spectrum is adjusted by adding a violet pigment. For example, U.S. Pat. No. 6,280,890 B1 discloses a blue colorant consisting of a copper phthalocyanine pigment and a dioxazine violet pigment. The dioxazine violet pigment accounts for 0.5 to 7% by weight of the total weight of the blue colorant. However, when a dispersion of the violet pigment is mixed into a dispersion of the copper phthalocyanine blue pigment, balance of the dispersion system is lost in many cases. Thus, it is generally difficult to obtain a stable blue colored composition.